1. Field of the Invention
The present invention relates to an encoder in which the relative displacement between the encoder scale and the encoder head is detected.
2. Description of the Related Art
Encoders are composed of an encoder scale and an encoder head and are designed to generate periodic signals of different phases in order to detect the relative displacement between the encoder scale and the encoder head. An output of the encoder may be supplied to a data-processing circuit that counts cycles and may therefore be used to detect the moving direction, position, displacement and displacement speed of an object. Such an encoder is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 6-26817.
FIG. 9 shows an exemplary configuration of the conventional encoder. As shown in FIG. 9, a resistor 103 is connected in series with a light source 101 in order to supply a fixed current. The light source 101 emits light, which is applied to a scale 103 that has an optical pattern of a specific periodicity and moves relative to the light source 101. Light passing through the scale 103, light reflected by the scale 103, light diffracted at the scale 103, or light scattered at the scale 103 is applied to light-receiving elements 104A and 104B so that the distance the scale 103 has moved may be detected from the changes in intensity of the light. The light-receiving elements 104A and 104B are arranged such that their output signals may have a phase difference of 180°.
The output of the light-receiving element 104A is connected to a current-to-voltage conversion means composed of a resistor 105A and an operational amplifier 106A. The output of the light-receiving element 104E is connected to a current-to-voltage conversion means composed of a resistor 105B and an operational amplifier 106B. The outputs of this current-to-voltage conversion means are connected to a subtraction means 107. Further, a reference voltage VREF (108) is applied to the operational amplifiers 106A and 106B and used as a reference value for cycle signals.
The photocurrents generated, as electrical signals, by the light-receiving elements 104A and 104B are input to a current-to-voltage conversion means. The current-to-voltage conversion means converts the photocurrents to voltage signals VPA and VPAB, respectively. The voltage signals VPA and VPAB each contain not only an AC component that represents the intensity change of the light reflected by the scale 103, but also a DC component and noise. Note that the DC component pertains to a light component that has a fixed intensity. The subtraction means 107 performs operation of VREF−(VPAB−VPA) in order to remove the DC component and the noise. A cycle signal VA that represents the intensity change only is thereby obtained. The encoder is configured to output several encoder signals such as electrical signals obtained by performing such a process as explained with reference to FIG. 9.